Liquid sulfur produced by the Claus sulfur recovery process contains significant amounts of dissolved hydrogen sulfide (H2S). The release of this H2S from the sulfur presents a number of serious safety and environmental concerns, ranging from explosions and toxic personnel exposure to localized nuisance odors. The preferred method for coping with these problems is to remove or “degas” the H2S from the liquid sulfur product prior to its subsequent storage, handling, and/or forming.
The process chemistry for H2S in Claus-produced liquid sulfur can be summarized as:                H2S present in the Claus process gases is at a significant partial pressure within a Claus plant and is thus dissolved in the liquid sulfur produced in the Claus sulfur condensers.        This dissolved H2S is subsequently liberated in downstream storage facilities when the sulfur cools and is exposed to lower atmospheric pressure conditions where the partial pressure becomes essentially zero.        
The equilibrium concentration of H2S in liquid sulfur under atmospheric conditions is quite low (<10 ppmw) which is much less than the H2S content of liquid sulfur produced from the Claus process (≈200-400 ppmw). Therefore, the natural result is the liberation of H2S gas. The purpose of degassing is to remove the H2S in an accelerated and controlled manner, and then properly dispose of it in a safe location and in an environmentally-friendly manner. There are five main reasons for degassing liquid sulfur:                1. Reduce Toxicity—H2S is an extremely toxic gas that is immediately fatal at concentrations greater than 1000 ppmv.        2. Reduce Explosive Hazards—H2S forms an explosive mixture in air at concentrations from 3.4 to 46 vol % at normal sulfur storage temperatures. This level is readily achieved in the headspace of downstream sulfur storage equipment if the sulfur is not degassed and/or the equipment is not properly vented.        3. Reduce Emissions—Undegassed liquid and formed sulfur products emit H2S to the atmosphere and give off noxious odors. Degassing the sulfur allows the H2S to be recovered and properly processed.        4. Improve Formed Sulfur Product—Solid sulfur formed from degassed sulfur is less prone to fracture. The presence of H2S gas in the formed product produces voids and surface imperfections that weaken the solid, making it more susceptible to breakage during handling and transport. This creates sulfur dust and releases the malodorous H2S.        5. Reduce Corrosion—H2S is corrosive to carbon steel, especially in a wet environment, and can cause corrosion in storage equipment and piping, as well as trucks, tank cars, and ships. Degassing the liquid sulfur helps to reduce corrosion in all downstream devices.        
Since H2S is slightly heavier than air, it can accumulate in confined spaces instead of being readily dispersed. This property will exacerbate the environmental concerns expressed above, especially with regards to personnel exposure.
There is no world-wide, universally accepted standard for degassed liquid sulfur. Sulfur degassing standards are expressed as total (H2S+H2SX) ppm by weight as H2S. The first degassed sulfur standards were developed in western Canada to address problems encountered when shipping large volumes of sulfur in rail tank cars. Contemporary standards are 30 ppmw in western Canada and 10 ppmw for Europe. China has also adopted a 10 ppmw standard. There are no statutory requirements in the United States, but many facilities reduce the content to satisfy particular customers of their liquid sulfur product.
Although the process description in the preceding section is nominally correct, the exact details and mechanisms of the chemistry of H2S exchange with liquid sulfur are much more complicated. Understanding the various chemical reactions and the appearance of intermediate chemical species is crucial to understanding the sulfur degassing process. More importantly, knowledge of this chemistry can be exploited to develop suitable degassing strategies. This section briefly describes the process chemistry of these physio-chemical interactions.
Pure elemental liquid sulfur at atmospheric pressure and at a temperature nominally above its melting point (245F), generally exists as an S8 molecule in a ring structure. At higher temperatures (>300F), a significant fraction of the S8 rings convert to a straight-chain structure. When elemental liquid sulfur exists as a straight-chain molecule, it can form polymers of varying chain length, sometimes on the order of thousands of units. For the general purposes, the chemical reaction can be written as:

It is the presence of the polymeric form at normal Claus condenser temperatures that complicates the physio-chemical interactions between H2S and liquid sulfur.
There is an equilibrium relationship between H2S in the vapor phase (either in the Claus tail gas or in the headspace of liquid sulfur storage equipment) and H2S dissolved in liquid sulfur. As with most gas solubility relationships, this dissolution is reversible and depends on the partial pressure of the H2S in the vapor phase and the temperature of the liquid sulfur. The equilibrium exchange of H2S between the vapor and liquid phases is relatively fast (eq-2). However, once the H2S is dissolved in the produced liquid sulfur, it can react with polymeric liquid sulfur to form polysulfide compounds (eq-3).

The formation of the polysulfide is relatively fast under Claus conditions, while the decomposition of the polysulfide back to H2S is relatively slow. The equilibrium distribution between the sulfide and the polysulfide forms is highly temperature dependent: the higher the temperature, the greater the fraction in the polysulfide form (see Table 1). At a temperature of 320F, the equilibrium H2S/H2SX ratio is close to 0.6. At 257F, the ratio is more than 3. This means that the H2S in the hot Claus gas is rapidly dissolved into the liquid sulfur and then quickly forms polysulfides. As the sulfur cools and the H2S partial pressure is reduced in the downstream storage equipment, there is a slow natural release of H2S from the liquid sulfur. The H2SX in the liquid gradually equilibrates back to H2S (eq-3 in reverse). Then the dissolved H2S that is in contact with the vapor space rapidly evolves out of solution (eq-2 in reverse). H2S dissolved in liquid that is not in contact with the vapor space will not be able to equilibrate with the vapor and evolve out of solution.
The liquid sulfur from a Claus SRU is usually collected in a storage tank, vessel, or below-grade concrete pit. This containment equipment usually operates under an air atmosphere. Initial studies concerning the influence of air were based on rail car studies that observed that the degassing rate was directly proportional to the liquid surface area and the H2S content.
Alberta Sulphur Research Ltd. (ASRL) research has shown that there a number of reactions between the dissolved sulfides and oxygen. Because of the nature of the testing systems and the difficulty of accurately measuring all of the reaction species in each the phases, the extent of the contribution of each of the oxidation reactions is difficult to quantify. However, reasonable qualitative assessments have been made based on the observed behavior.
The sulfides can be partially oxidized with air (oxygen) according to eq 4 & 5 or more completely oxidized via equation 6 & 7 below.

Experiments confirm the appearance of SO2 in the vapor, but only when there is also H2S/H2SX present in the liquid. Once the H2S evolves and is swept away, no more SO2 is formed. This rules out direct oxidation of elemental sulfur:

It is also speculated that the Claus reaction occurs, but only to a small extent. This could occur via eqs 9 & 10

From the preceding description of the reactions of oxygen, hydrogen sulfide and sulfur at conditions normally encountered in industrial Claus sulfur recovery units, it can be seen that degasification using air is a convenient and practical approach. There are several commercially-proven processes which exploit this concept. The feature of the majority of these other processes is the source of air is from an external supply at pressure (i.e., through the use of an air compressor). Of these processes several different approaches are used to provide the intimate contact of air and the sulfur (e.g., auto-recirculation boxes, spargers, packed beds, spray towers). The main disadvantage of these existing processes is that the quantity of air used to degas the sulfur exceeds the minimum required to control the sweep air rate hydrogen sulfide lower explosive limit which leads to excess emissions of sulfur.
It is noted that for degasification of sulfur that the temperature of the process is important, decomposition of hydrogen polysulfides is best promoted by cooling the liquid sulfur to 265-285F to shift the H2SX⇄H2S equilibrium distribution towards H2S, while still keeping the liquid sulfur safely above its melting point (245F). Also, cooler sulfur temperatures can significantly reduce sulfur viscosity, resulting in better liquid/vapor interfacial contact (i.e., liquid sulfur/air).